I thought they were dropped in favour of Borg, Borg and more Borg.
Not necessary in the order, but here are some thoughts:
I think a Vadwaar arc should have included them recovering an ancient weapon they hid away somewhere and using it to try and reclaim their territory. Voyager must then unite several races together to fight them in order to survive, but with the mini-Federation unstable and prone to in-fighting they aren't ready for what is coming. It would probably be a two-parter.
They recover a small Vadwaar ship with a hot babe in it, who Harry falls for, only to have her turn evil and try to seize control of the ship. Ultimately she is defeated by Seven's nanoprobes, as they wrestle in a vat of thick mud-like coolant.
Giving aid to a freighter packed with refugees from a Vadwaar attack, Voyager then comes under attack and defeates the strike ship and take four survivors prisoner (only two of who have speaking parts). The refugees demand the prisoners be handed over for trial and execution for war crimes. During the time, Neelix bonds in a creepy kind of way with a child from the refugee ship, whilst Seven uses one of the Vadwaar to examine their ship in more detail, and begins to form a friendship with them. Ultimately the refugees kills the one the Seven befriended.
Janeway and a Vadwaar are stranded on a barren moon, where they must put their differences aside and work together to survive. Seven uses astrometrics to find them. They have to battle through half a dozen Vadwaar ships to get Janeway back though.
They come across a Vadwaar station onboard which the crew are sick and dying. The Doctor and Seven (whose nanoprobes protect her) have to beam over to find a cure. Just as they do a Vadwaar fleet arrives on the scene and attacks Voyager, thinking they poisoned the station.
